1. Field of the Invention
The present invention is directed to probe-based instruments and, more particularly, relates to a method and apparatus for driving a cantilever of such an instrument using acoustic radiation pressure generated by an ultrasonic actuator.
2. Description of Related Art
Several probe-based instruments monitor the interaction between a cantilever-based probe and a sample to obtain information concerning one or more characteristics of the sample. For example, scanning probe microscopes (SPMs) typically characterize the surface of a sample down to atomic dimensions by monitoring the interaction between the sample and a tip on the cantilever probe. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.
The atomic force microscope (AFM) is a very popular type of SPM. The probe of the typical AFM includes a very small cantilever which is fixed to a support at its base and which has a sharp probe tip attached to the opposite, free end. The probe tip is brought very near to or into contact with a surface of a sample to be examined, and the deflection of the cantilever in response to the probe tip""s interaction with the sample is measured with an extremely sensitive deflection detector, often an optical lever system such as described in Hansma et al. U.S. Pat. No. RE 34,489, or some other deflection detector such as strain gauges, capacitance sensors, etc. The probe is scanned over a surface using a high-resolution three axis scanner acting on the sample support and/or the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansma et al. U.S. Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,226,801; and Elings et al. U.S. Pat. No. 5,412,980.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. In contact mode operation, the microscope typically scans the tip across the surface of the sample while keeping the force of the tip on the surface of the sample generally constant. This effect is accomplished by moving either the sample or the probe assembly vertically to the surface of the sample in response to sensed deflection of the cantilever as the probe is scanned horizontally across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Alternatively, some AFMs can at least selectively operate in an oscillation mode of operation such as TappingMode,(trademark) In TappingMode(trademark) the tip is oscillated at or near a resonant frequency of the cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample.
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
One potentially problematic characteristic of AFMs and other probe-based instruments lies in the technique employed to provide an external force to deflect or oscillate the instrument""s cantilever. In an AFM, the cantilever is typically oscillated using a piezoelectric drive, often known simply as a piezo drive. Referring to FIG. 1A by way of example in this type of system, the typical probe 20 includes a cantilever 22 that extends outwardly from a substrate 26 coupled to a piezoelectric drive 24 via a probe mount 27. Probe 20 also includes a tip 28 that is provided on the opposed, free end of the cantilever 20. The piezoelectric drive 24 can be selectively excited by a signal generator 29 to move the cantilever 22 up and down relative to a sample 30. When the instrument is configured for an oscillating mode of operation, the drive voltage is applied to the piezoelectric drive 24 to drive the cantilever 22 to oscillate at a frequency that is dependent upon the frequency of the drive voltage. This frequency is typically at or near the cantilever""s resonant frequency, particularly when the instrument is operated in TappingMode(trademark).
Such a traditional piezoelectric drive necessarily acts only on the base of the cantilever, not on the free end portion. It therefore must apply substantially greater forces to the cantilever to obtain a given deflection magnitude at the free end than it would if it were to apply forces directly to the free end or even to the body of the cantilever. This inefficiency limits the range of applications for this common type of piezo-electrically-driven probe.
For instance, the piezoelectric drive shown in FIG. 1A works well in air because the typical AFM cantilever can be excited to resonance in air relatively easily. This characteristic is quantified by the xe2x80x9cquality factorxe2x80x9d of a resonance of the cantilever. The quality factor, Q, denotes the sharpness of a cantilever""s resonance curve as denoted by the ratio: f0/xcex94f, where f0 is the resonant frequency and xcex94f is the bandwidth between the half-power points of the curve as reflected by the half-peak amplitude points 41a and 41b on the curve 40 in FIG. 2A. The curve 40 demonstrates that the typical cantilever operating in air has a Q of 100-200 or even higher. The Q of a cantilever resonance is a measure of how much gain the resonance provides in an oscillating system. A resonance with a large Q can be excited to relatively large cantilever oscillation amplitudes with relatively small excitation forces. For operation in air or other gaseous environments, the cantilever typical piezoelectric drive usually has ample excitation force to drive the cantilever to produce a resonance peak 42 that is easily identified and distinguished from other, parasitic resonance peaks such as those of the mounts for the cantilever and the piezoelectric drive and the piezo drive itself (note the much smaller peaks 42a, 42b, etc. denoting these parasitic resonances).
Conversely, a cantilever operated in liquid such as water has a dramatically lower Q because the liquid damps the oscillating cantilever. In fact, the typical cantilever operating in water has a Q of less than 30 and even less than 10. As a result, the typical piezoelectric drive does not have enough gain to excite the cantilever sufficiently to produce a resonance peak that is easily located and differentiated from parasitic resonances. This effect is discussed below in conjunction with FIG. 2B.
Specialized cantilever drives are available that act along the length of the cantilever rather than only on the base. One such drive is the so-called magnetic drive. Referring to FIG. 1B, the typical magnetic drive system 50 has a magnetic cantilever 52 that is driven by an electromagnetic drive. The cantilever 52 has a fixed base rigidly attached to a support 54 and bears a tip 56 on its free end that interacts with sample S. The cantilever is also rendered magnetic by coating one or more of its surfaces with a magnetic layer 58. The electromagnetic drive comprises at least one electromagnet coil 60 spaced from the cantilever 52. The coil(s) can be energized by a controller 62 including a signal generator to impose a variable a magnetic field on the magnetic layer 58. The magnetic field produces a torque on the cantilever 52 of a magnitude that increases with the amplitude of the applied magnetic field acting on the layer 58. By controlling the amplitude of the applied magnetic field, the cantilever 52 can be deflected as desired while the tip 56 interacts with the sample S. This deflection is monitored by a photodetector 66 receiving reflected light transmitted by a laser 68. In the usual case in which the magnetic drive 60 is controlled to maintain a specified characteristic of cantilever deflection constant during scanning, an output signal related to the amplitude of the signal provides an indication of a surface force applied to the probe. A magnetic drive system having these characteristics is described in greater detail in U.S. Pat. No. 5,670,712 to Cleveland, the subject matter of which is hereby incorporated by reference by way of background.
A magnetic drive system has inherent limitations that considerably restrict its range of applications. For instance, it requires a special magnetically coated cantilever and, accordingly, cannot be used in applications in which the cantilever is not capable of being coated with a magnetic material. It also is not usable in applications in which magnetic properties of the sample and/or the environment cause unwanted deflection of the cantilever and produce errors into the measurements. The practical operating ranges of the magnetic drive system are also limited. A typical magnetic drive coil may operate with a current exceeding an amp and result in a cantilever deflection on the order of 1-100 nm at the cantilever resonance frequency. Even at this coil current, the heat load generated can cause thermal drift errors in the measurement of the AFM. The frequency range of the magnetic drive system is also limited by the inductance of the drive coil. Higher actuation forces can be achieved by using more loops in the drive coil, but this also increases the inductance and limits the maximum operating frequency. With the limits of inductance and maximum heat load, the typical magnetic drive operates with less than 50 kHz and with oscillation amplitudes of less than 30 nm. For example, the MAC-Mode(trademark) magnetic drive system, sold by Molecular Imaging, advertises an operating range of 5-30 kHz and a maximum amplitude of 30 nm.
Another instrument having a cantilever driven remotely from its base utilizes the so-called acoustic drive. Referring to FIG. 1C, in an instrument 70 of this type, a cantilever 71 and a piezoelectric drive 72 are mounted on a common head 74 in a spaced-apart relationship. The head 74 is mounted above a fluid cell 76 by mounting balls 78 or other supports so that the cantilever 71 extends into the fluid cell 76 so as to interact with a sample (not shown) in the cell. The piezoelectric drive 72 can be excited by an signal generator 80 to generate acoustic waves that propagate through the glass walls of the fluid cell 76, through the fluid in the cell 76, and onto the cantilever 71, causing the cantilever 71 to oscillate. An acoustic drive having these characteristics is disclosed, for example, in Putman et al in xe2x80x9cTapping Mode Atomic Force Microscopy in Liquidsxe2x80x9d Applied Physics Letters 64: 2454-2456.
Acoustic drive has distinct disadvantages that limit its effectiveness. For instance, the acoustic energy also impinges on many other components of the system, such as mounts for the cantilever and the piezoelectric drive, the fluid cell, and even the fluid exciting, resonances in those components. These resonances can be difficult to distinguish from the cantilever resonance. The acoustic drive also has sufficient actuation force at a limited selection of operation frequencies and it can be a challenge to match the cantilever resonance with the operation frequency of the acoustic actuator. If a user selects a resonance that does not overlap with the cantilever resonance, the measurements may be unstable.
An ultrasonic force microscope (UFM) is a scanning probe microscope that uses high frequency acoustic waves to image the mechanical properties of a sample, often showing sub-surface contrast. Specifically, referring to FIG. 1D, a UFM 90 includes a cantilever 91 having a base fixed to a stationary support 94, a sample support 92 located beneath the cantilever 91, and on an XYZ scanner 96 that supports the sample support. An ultrasonic actuator 98 such as commercial ultrasound transducer mounted on the bottom of the sample support 92 and is excited by an RF voltage from an RF signal generator 100. The ultrasonic actuator 98 is relatively large (typically a centimeter or more in diameter) with a resonant frequency often in the low-MHz range. When it is excited by the RF signal generator 100, it generates ultrasonic waves that impinge over a broad area of the sample S. Some of the incident ultrasonic energy is reflected or absorbed, and some penetrates the sample S and then impinges on the cantilever 91, causing the cantilever 91 to deflect away from the sample surface. The magnitude of the cantilever deflection is related to the percentage of the energy that penetrates the sample S and, accordingly, the, reflects variation in sample properties such as density. Accordingly, as the sample S is scanned relative to the probe using the scanner 96, variations in cantilever deflection can be detected to provide information concerning the sample. In addition, while UFMs have been in use for almost a decade, no one has adapted an ultrasonic device as a general purpose cantilever actuator capable of deflecting the cantilever at a wide range of frequencies.
Turning to FIG. 2B mentioned above, the plots demonstrate the frequency response of a typical AFM cantilever to excitation. The curve 44 plots the actual or true response of a relatively short and thick cantilever in water as determined by a known process called a xe2x80x9cthermal tune.xe2x80x9d A thermal tune measures the natural intrinsic motion of the cantilever in response to the temperature of its surroundings. Basically, the xe2x80x9cheat bathxe2x80x9d that surrounds the cantilever provides the energy to naturally oscillate at a very small amplitude, usually sub-nm. Since the cantilever oscillation amplitude due to the thermal energy is so small, thermal tunes cannot be used for image data acquisition, but they do provide a very clean representation of the true oscillatory response of the cantilever. The curve 46 plots the detected response of the same cantilever as it is driven acoustically by a piezoelectric drive (FIG. 1C). The true response as denoted in curve 44 has a sharp peak 48 at the fundamental resonance of about 15 kHz. However, when the cantilever is driven acoustically by a piezoelectric drive, fluid damping and other effects reduce that response to the point that the cantilever resonance peak cannot be differentiated from parasitic resonance peaks.
Hence, the need has arisen to provide a probe-based instrument that has an actuator that drives the cantilever 50 as to produce a xe2x80x9ccleanxe2x80x9d frequency response, preferably by driving the cantilever body rather than the base, but that is versatile in bandwidth/or types of measurements.
The need has also arisen to provide an improved method of driving a cantilever of a probe-based instrument.
In accordance with a first aspect of the invention, one or more the above-identified needs is met by providing a probe-based instrument having a cantilever that is deflected by directing acoustic waves onto the body of the cantilever rather than by moving the base of the cantilever. The cantilever is deflected by a second order force, also known as an acoustic radiation force, generated by beams of ultrasonic energy generated by an ultrasonic actuator such as a zinc oxide transducer. The ultrasonic actuator is supplied with an oscillating RF voltage that may be continuous or varied in a quasistatic manner to apply a constant or changing force to the cantilever. The RF voltage may also be modulated at any frequency from DC to many M:Hz, thus providing an ideal drive force for oscillating the cantilever over an extremely wide range of frequencies. Driving the body of the cantilever with an ultrasonic actuator produces a much higher localized force than can be achieved through the use of a traditional piezoelectric actuator and, accordingly, permits a xe2x80x9ccleanxe2x80x9d frequency response where the resonance peak is easily identified and differentiated from parasitic resonance peaks. This, in turn, dramatically improves the accuracy, precision, and stability of the measurement, and increases the system""s bandwidth, particularly when the cantilever operates in a liquid. The method implemented by the invention can be used to actuate cantilevers with arbitrary shapes and materials, eliminating the requirement for magnetic or piezoelectric coatings on the cantilever. The method and system of the preferred embodiments also are useful in imaging in liquids and quantitative measurements of surfaces and molecular-scale samples in liquids. The method and system of the preferred embodiments also are useful in AFM measurements in other fluids including air.
The improved frequency response of the ultrasonic actuator of the preferred embodiment also yields a dramatically higher bandwidth than traditional piezoelectric actuators, rendering them useful in a variety of applications and with a variety of cantilevers beyond those available with conventional piezoelectric actuators.
The beam is preferably xe2x80x9cshapedxe2x80x9d, i.e., manipulated to limit unwanted propagation in directions other than toward the cantilever, so that ultrasonic energy impinges at least primarily on the cantilever. Two suitable techniques for shaping the beam are focusing and collimation. Ultrasonic beams can be focused on the cantilever using a Fresnel lens or another focusing device located between the ultrasonic actuator and the cantilever. Collimation requires only that the ultrasonic actuator be suitably sized, positioned, and driven to reduce beam divergence sufficiently to achieve the desired effect.
Cantilever deflection may be measured by a conventional photodetector, in which case the photodetector, a laser, and the ultrasonic actuator are all preferably positioned on a common side of the cantilever opposite the sample support. Cantilever deflection may also be detected using another device such as a simple interferometer located over the cantilever body.
These and other features and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications